(1) Field of the Invention
The present invention relates to a welded can having a coated side seam and a process for the preparation thereof. More particularly, the invention relates to a welded can excellent in the corrosion resistance and processability, in which a coating of a thermosetting resin and a thermoplastic resin is formed on a seam by one step and in this coating, one of the two resins is present as a continuous phase and the other resin is present as a dispersed phase, and also to a process for the preparation of such can.
(2) Description of the Prior Art
As the conventional process most popularly adopted for the manufacture of can bodies, there can be mentioned a process comprising forming a metallic can blank cut in a predetermined size into a cylinder, lapping both the edges of the blank and bonding the lapped edges to form a side seam by welding, soldering or using an adhesive.
In a can body prepared according to this process, the cut edge of the blank is exposed on the inner face side of the can body, and in order to prevent corrosion of the can blank and control dissolution of the metal into the packed content, it is very important to coat this exposed cut edge.
Various processes for coating and protecting side seams, especially cut edges, have been heretofore proposed. As the process capable of attaining considerable effects, there can be mentioned a process in which the cut edge of a can blank is coated and protected in advance with a tape of an adhesive such as a polyamide. This process can be applied to the manufacture of bonded seam cans, but this process cannot be practically utilized if the seam is exposed to a very high temperature as in the case of welded seam cans.
As means for coating and protecting the side seam of a welded seam can, there is known a process in which a paint in the form of a solution or powder is coated on the inner or outer face side of the seam after formation of a can body. However, known paints used for this purpose are insufficient in the combination of adhesion to the side seam, the barrier property against corrosive components and processability, for example, adaptability to the double-seaming operation, and the object of completely coating the stepped cut edge in the side seam cannot be satisfactorily attained by these known paints.
For example, a coating material comprising a thermoplastic resin has excellent processability, but the thermoplastic resin is poor in adhesion to the side seam and insufficient in the above-mentioned barrier property. Accordingly, a side seam coated with a thermoplastic resin is readily corroded by the content or the like and defects such as sulfide blackening are caused.
A paint comprising a thermosetting resin is excellent in the adhesion to the side seam and the barrier property to corrosive components, but has inferior processability. In a can body having a side seam coated with a thermosetting resin, dissolution of the metal from the side seamed portion is often observed.
When these known paints are used, whether they comprise a thermosetting resin or a thermoplastic resin, they flow in the form of a solution or in the molten state so that the stepped portion of the cut edge is levelled. Therefore, the coating is not present in the angular portion of the cut edge or the thickness of the coating is extremely reduced in this portion. Furthermore, air bubbles are readily included in the coating of the stepped portion. Thus, it is substantially impossible to coat the cut edge of the can blank completely.
If there is present an exposed metal portion in a can body, a defective can such as a hole-pierced can or a hydrogen-swollen can is formed according to its content, and furthermore, the flavor of the content is drastically degraded.